[patch] Avoid the use of mmap as buffer for stdio streams

[Arjan van de Ven]

Ulrich Drepper wrote:
> What code is _really_ affected by this? 

I noticed it while stracing the new version of irqbalance that I'm 
developing. That needs to fopen/read/fclose 3 virtual (proc/sysfs) 
files every iteration; it does the munmap 3 times. I know that a whole 
lot of the desktop stack (hal, gnome-power-manager etc) do this as 
well, adding up to a few dozen such things every second.

Many of the "traditional" unix tools like tar and grep don't use stdio 
so those don't count; that's one point for your argument.

A case that I found that does count in my favor is nautilus; it uses 
stdio for everything and does a lot of fopen .. read some .. fclose 
sequences when going into a new directory
(I suspect for reading the first bits of each file to determine what 
icon to use)


> We are using mmap for a number 
> of reasons, one of which is that it reduces fragmentation on the heap.

... by trading virtual address space fragmentation some ;)

  > It works well and so far I don't see any reason to change this.  I've
> not seen programs which constantly and with high frequency fclose() files.
I bet if you look at java ...

anyway, one of the things against the mmap implementation is that it 
allocates 4K for a buffer it uses only 1Kb of. Would you at least 
consider changing the default buffer to be PAGE_SIZE if you keep 
insisting on using mmap? To avoid the wasting of this 75% of the 
memory that's entirely not used....